DMF-induced emission of an aryl-substituted pyrrole derivative: a solid thermo-responsive material to detect temperature in a specific range

The aryl-substituted pyrrole derivative, 4-(2,5-diphenyl-1H-pyrrol-1-yl)benzoic acid (TPPA), was found to have a controllable fluorescence in the solid state due to its propeller-shaped molecular design and adjustable molecular packing. Melding a carboxylic acid unit with triphenylpyrrole turned the typical aggregation-induced emission (AIE) luminogen into a luminogen that only emits intensely in a certain aggregation form—i.e., when it is crystallized from dimethyl formamide (DMF). A thermal responsive solid material was developed by exploiting this property. Its fluorescence remained almost unchanged at relatively low temperature (<70 °C) but was greatly quenched when the temperature reached ∼85 °C. Based on thermal analysis and microscopic investigations, this distinct quenching effect was attributed to the thermal volatilization of DMF. The DMF loss in the crystal lattice inevitably loosened the molecular packing, thus opening a non-radiative relaxation pathway to quench the fluorescence. The rapid response (less than 30 s), high selectivity (loss of fluorescence at >85 °C), good reversibility and solvent-free procedure make TPPA a thermo-responsive material for use in temperature monitoring devices.